This invention relates to oscillators, in particular oscillators whose operating frequency can be varied in response to an input.
There is a need for low-cost and highly efficient transmitters for a wide class of radio communication and measurement devices. Phased-Locked-Loops (PLLs) are integral to radio frequency (“RF”) systems for generating an output signal having a predetermined frequency and/or phase relationship with a reference signal. The PLL comprises an oscillator device for generating a desired frequency signal. Such an oscillator device may be a digitally controlled oscillator (DCO) or a voltage controlled oscillator (VCO). In systems that use PLLs for the direct modulation of a transmit signal, e.g. continuous-wave (CW) FM in radar or polar modulations, the linearity of the oscillator gain Kv becomes very important. Nonlinearity in Kv can cause a drop in the modulation accuracy as well as corrupting the spectrum of the transmit signal.
In an example application, polar modulation transmitters have seen increased demand in recent years due to low cost and higher efficiency. Polar modulation requires a highly linear phase modulator, and especially so for a wideband application. This linearity is at least partially related to the oscillator gain linearity. However, oscillator gain in many actual designs suffer from nonlinearity because of dependence of the gain on the frequency. In some systems, nonlinearity in the oscillator gain directly affects the modulation accuracy (EVM) and transmit spectrum (ACP).
Prior systems and methods to improve the linearity of the oscillator gain include a pre-distortion method (look-up table based nonlinearity compensation), delay line-based compensation and analogue and/or digital real-time frequency compensation circuits. These prior systems and methods can be circuit intensive, costly, and have power and area penalty.
Thus there is need for improving the linearity of oscillator gain in an easier, more efficient and economic manner.